1. Field of the Invention
This invention relates to new isocyanato-isocyanurates based on isocyanatobenzyl cyclohexyl isocyanates optionally alkyl-substituted on the aromatic ring, to a process for their production by partial trimerization of these diisocyanates and to the use of the isocyanato-isocyanurates in optionally blocked form as the isocyanate component in the production of isocyanate polyaddition products, preferably polyurethane lacquers.
2. Description of the Prior Art
It is known that isocyanuric acid derivatives can be produced by the catalytic trimerization of organic isocyanates. The trimerization of organic diisocyanates containing isocyanate groups of identical or similar reactivity only gives triisocyanates to a predominant extent when the formation of relatively high molecular weight products is avoided or suppressed by premature termination of the reaction.
For the production of monomer-free isocyanato-isocyanurates (which are of interest as soluble, polyfunctional and, by virtue of their high vapor pressure, as physiologically acceptable isocyanates, particularly for lacquer applications), the monomeric diisocyanate has to be removed from these isocyanurate-containing products obtained by partial trimerization in a second process step, for example by thin-layer distillation.
By contrast, diisocyanates containing two isocyanate groups of different reactivity may, in certain cases, be directly reacted in a single step to form soluble isocyanato-isocyanurates. Examples of selective trimerization reactions such as these can be found in British patent No. 949,253 and in DE-AS No. 1,203,792. In these processes, 2,4-tolylene diisocyanate is used as the starting material and is trimerized either in the absence of solvents or in solution to form. isocyanato-isocyanurates. The direct reaction to form low-monomer, sufficiently thin-flowing products is only possible in solution using at least 50% by weight of solvent. As explained by E. Windemuth (Kunststoffe, Vol. 57, 1967, pages 337-343), the trimerization of 2,4-tolylene diisocyanate normally gives products of very high molecular weight providing the reaction time is sufficiently long: low-monomer products predominantly containing triisocyanates only being obtainable where special trimerization catalysts are used. Isocyanato-isocyanurates containing aromatically bound isocyanate groups are not really suitable for light-stable lacquer systems because products such as these have a tendency towards yellowing.
Mixed isocyanato-isocyanurates produced from aliphatic and aromatic diisocyanates, as described for example by L. Havenith (Paint Manufacture 38, 1968, page 33) and W. Wieczorreck (Farbe und Lack 75, 1969, page 318), turn yellow distinctly more slowly. These polyisocyanates based, for example, on tolylene diisocyanate and 1,6-hexamethylene diisocyanate may be processed to form films having relatively short drying times, so that lacquering can be carried out on the assembly-line principle with short cycle times, as in the case of the purely aromatic isocyanato-isocyanurates. However, the production of mixed isocyanato-isocyanurates containing free aromatic and aliphatic isocyanate groups involves several stages, as described in DE-PS No. 1,670,667.
Lacquer systems based solely on aliphatic isocyanates show extremely high light stability. Corresponding isocyanato-isocyanurates, which are widely used as light-stable surface lacquers in either solvent-free or solvent-containing form, may be produced from 1,6-diisocyanatohexane and from 1-isocyanato3,3,5-trimethyl-5-isocyanatomethyl cyclohexane (isophorone diisocyanate), for example by the processes described in EP No. 0,010,589 and in DE-PS No. 2,325,826, respectively.
Another criterion for the suitability of polyisocyanates for use as isocyanate component in polyurethane lacquers is their compatibility with apolar solvents, for example aromatic and aliphatic hydrocarbons. Isocyanato-isocyanurates produced in particular from aromatic diisocyanates, for example tolylene diisocyanate, show distinct disadvantages in this respect because they can only be diluted to a limited extent with aromatic and aliphatic hydrocarbons.
In DE-OS No. 2,414,413, it is recommended that the polyisocyanate component be additionally modified with fatty alcohols to improve its compatibility with apolar solvents and to reduce its content of unreacted starting diisocyanate. Although this modification does produce a significant improvement in solvent compatibility, it is attended by the disadvantage that isocyanate groups are additionally consumed and are therefore no longer available for the formation of crosslinking sites, thereby reducing the functionality of the polyisocyanates.
The object of the present invention is to provide new isocyanato-isocyanurates which can be inexpensively produced by a technically simple process and which show good to very good lacquer properties or property combinations.
This object is achieved by the provision of the new isocyanato-isocyanurates described in detail hereinafter and by the process for their production.
By virtue of the process according to the invention, it is possible in a single step to produce low-monomer isocyanato-isocyanurates which contain almost exclusively aliphatically bound isocyanate groups and, accordingly, give lacquer films characterized by high light stability. In addition, it is also possible by the process according to the invention to produce in a technically simple manner low-monomer isocyanato-isocyanurates of the type which contain both aliphatically and also aromatically bound isocyanate groups and which, accordingly, may be used as lacquer-grade polyisocyanates having graduated reactivity and improved light stability. In addition, some of the products obtained by the process according to the invention show outstanding compatibility with apolar solvents.